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Summary of Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($)
$ in Thousands
Jan. 01, 2018
Dec. 31, 2017
Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Adjustments to equity   $ 3,104,950 $ 3,199,405
Comprehensive Income | Subsequent Event | ASU 2016-01      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Adjustments to equity $ (86,572)    
Retained Earnings | Subsequent Event | ASU 2016-01      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Adjustments to equity $ 86,572